I've just looked on my mortgage sourcing system and in principle you could borrow up to 75% of the property value, or £525,000. Once you clear the existing mortgage, you will have £400,000 as a deposit for the new property. I would be very surprised if you could borrow 6 times your income, but I would expect 4 times to be more likely. This would mean a figure of £144,000 + £400,000 deposit, or £544,000 in total. I know this isn't quite enough, but better than nothing.
There is actually a lender that would lend 80% on a buy to let basis, but the interest rate would be considerably higher. However, this would enable you to borrow £560,000, which would leave £435,000 as a deposit. This would then only require a mortgage of £155,000 to meet the target price of £600,000.